The Solar Balloon is a toy that has been around for a few years now. There are a number of sizes, but the one we show here is the largest - it is 50 feet long!
The toy is very simple - it consists of a black bag made of very thin (and therefore lightweight) plastic. The first step is to fill the Solar Balloon with air. Running around holding one end open does this effectively and quickly. Then you just wait...
The ideal weather conditions are a calm day, with bright sunshine. It actually works best if the air temperature is not too high. As the sun shines onto the black Solar Balloon, it starts to absorb heat, and the air inside slowly gets warmer. As this process continues, the bag starts to become lighter than the surrounding air. The principle is the same as a hot air balloon, although the temperature gradient is not so great in this case.
Once the Solar Balloon becomes buoyant, you can fly it into the sky, ideally attached to a thin thread so that you don't frighten local low flying planes!